1. Field of the Invention
The present invention relates to a ring oscillator formed on an integrated circuit such as a timer circuit for performing self refresh of DRAM (dynamic random access memory) and PSRAM (pseudo static random access memory), etc.
2. Description of the Related Art
As is shown in FIG. 4, a conventional ring oscillator formed on an integrated circuit such as a timer circuit for a semiconductor memory device comprises an odd number of inverters 111a, 111b and 111c, an inverter 111d, and a delay circuit 112. The inverters 111a, 111b and 111c are connected to each other so as to make a ring, and the inverter 111d and the delay circuit 112 are inserted in the ring of the ring oscillator. Accordingly, a signal outputted from the delay circuit 112 is inverted via the odd number of inverters 111a, 111b and 111c. Then the signal outputted from the inverter 111c is inputted into the inverter 111d. Therefore, the ring oscillator oscillates in a frequency in accordance with the time required for the passage of the signal through the odd number of inverters 111a, 111b and 111c, and the inverter 111d, and the time delayed in the delay circuit 112.
The oscillating frequency of a conventional ring oscillator is varied in accordance with the time delayed in the delay circuit 112. The time delayed in the delay circuit 112 is varied in accordance with the time period from the time when an output level of the inverter 111d becomes low (L) to the time when a terminal voltage level of a capacitor C charged via a resistor R becomes high (H). Therefore, the oscillating frequency can be varied by suitably changing the time constant of a circuit formed of the resistor R and the capacitor C.
The resistor R and the capacitor C are formed on the integrated circuit. Accordingly, the oscillating frequency of the conventional ring oscillator is determined based on the resistance value of the resistor R and the capacitance value of the capacitor C. The oscillating frequency of the ring oscillator formed on the integrated circuit is not correlated with the leakage current generated from a memory cell. Moreover, when the ring oscillator is used for performing self refresh of the memory cell, the oscillating frequency should be determined based on the case where the amount of the leakage current generated from the memory cell is the largest. Therefore, there is a disadvantage in that the ring oscillator unnecessarily performs the self refresh when the amount of the leakage current generated from the memory cell is not so large.